The concept of printing selected areas of a document in an infrared absorptive carbon black ink, and printing other areas of the document in an infrared transparent simulated black ink is previously taught in Researach Disclosure, No. 160, August 1977, (Vants Hants) GB "Printed Documents and the Detection of Markings Thereupon". By using an appropriate infrared emitter and detector, differentiation can then be made between the respective areas, and lack of authenticity of the document can be determined bassed on deviations from the original of that document.